As an inflator for an inflating type safety system of an automobile, in order to optimally protect a passenger in accordance with a position of a seat in a vehicle such as a driver side, a passenger side and the like, there are generally used various inflators. As the inflator, ones using a pressurized gas such as argon, helium or the like, as inflating means for an air bag are known. In such an inflator, since outflow of a pressurized gas is started by rupturing of a rupturable plate and an air bag is inflated and developed at the end, it becomes important to improve rupturability of the rupturable plate for improving an operational reliability of the inflator. Further, in order to satisfy a demand for size-reduction and weight-reduction, it is necessary to simplify a structure of the inflator as much as possible and satisfy demands for such as simplification of a manufacturing process, a reliability of an operation over a long period.
An invention relating to a stored gas inflator is disclosed in JP-A 2002-172995. In FIG. 2 of this invention, a main chamber 20 and a small chamber 18 are formed, a communication hole 26 and a small hole 28 are formed in a separation wall 24 positioned between these chambers, and a gas ejecting hole 14 and the communication hole 26 formed in the small chamber 18 are respectively mounted with rupturable plates (burst shims) 16 and 22. An initiator 30 is disposed in a pressurized atmosphere inside the small chamber 18, and there is a description that the rupturable plate can be ruptured even by an igniter with a small output in the specification of the publication. In this inflator, however, there are problems as follows.
In this inflator, in order to maintain a pressurized atmosphere inside the small chamber 18, it is necessary to seal the initiator 30 securely in order to keep air-tightness, not to let the gas inside the small chamber 18 leak from the initiator 30. However, in view of charging the gas with a high pressure, it is difficult to maintain the air-tightness over a period of 10 years or more which is a life period of a vehicle.
Further, in the paragraph 24 of the specification, there is a description that “P2 is approximately equal to (P1−Pm) or slightly smaller than the same”. Here, P2 represents a rupturing pressure for the rupturable plate 22, P1 represents a rupturing pressure for the rupturable plate 16, and Pm represents a charging pressure of a gas charged in the small chamber 18 and the main chamber 20. According to the contents of the disclosure, the temperature inside the small chamber 18 and the number of moles of the gas rise according to actuation of the initiator 30. Accordingly, when the pressure in the small chamber 18 becomes higher than that in the main chamber 20, there are no problems in that two rupturable plates 16 and 22 are simultaneously ruptured. However, if the rupturable plate 22 is ruptured first, the pressure inside the small chamber 18 escapes into the main chamber 20, so that a shock wave generated by the initiator 30 hardly acts on the rupturable plate 16. As a result, the rupturable plate 16 is not ruptured, and the air bag can not be inflated normally.
Furthermore, even though the initiator 30 is activated, there is not the rupturable plates 16 and 22 in extension line of a direction in which the shock wave travels, and accordingly, reliability is definitely poor when it is considered in view of rupture of the ruptrable plate.